The invention relates to a pitch bearing and more particularly the invention relates to a pitch bearing that includes a variable thickness resilient layer that produces bearing loading that promotes substantially constant bearing wear life.
Helicopters include a rotor hub that is driven by a rotor shaft. A plurality of rotor blades are connected to the rotor hub, and each rotor blade is linked to the hub by a member frequently referred to by those skilled in the relevant art as either a pitch hinge or a pitch link. For purposes of this description the member shall be referred to as a pitch joint. Each pitch hinge is used to make adjustments to the pitch or angle of attack of the blades to control the amount of lift generated as the rotor blades are rotated by the rotor hub. Each pitch hinge includes a number of pitch bearings that support the pitch hinge when it experiences an applied shear force and moment as the rotor hub is rotated.
Turning now to the schematic representation of a conventional pitch hinge illustrated in FIG. 1, the pitch hinge 10 has an inner body 12 with an inboard end 14 where the pitch hinge connects to the rotor hub (not shown) and an outboard end 16 where the pitch hinge connects to the respective rotor blade (not shown). The pitch hinge body is enclosed by an outer housing 18 that directly connects the pitch hinge to the rotor hub and rotor blade assemblies. The housing is partially illustrated in FIG. 1 and the conventional means for connecting the housing to the rotor blade and to rotor hub are not illustrated.
A pitch hinge elastic center 20 is located between the inboard and outboard ends 14 and 16. Conventional pitch bearings 22xe2x80x2 and 22xe2x80x3 are respectively located at the inboard and out board body ends and serve to support the pitch hinge as it experiences reactionary loading and applied rotational moments during use. For purposes of this disclosure, the elastic center of the pitch hinge is the location along the body of the pitch hinge where the reactionary loading forces produce no rotation. There is an applied moment at the elastic center. The location of the elastic center is dependent on a number of variables including the application environment for the pitch hinge, loading experienced by the pitch hinge and also the configuration and type of bearings used to support loading to the hinge. The moment applied about the elastic center is represented in FIG. 1 at the elastic center 20 by arrow 26.
The inboard and outboard pitch bearings 22xe2x80x2 and 22xe2x80x3 are substantially similar and include inner annular bearing seat 28xe2x80x2 and 28xe2x80x3, outer annular bearing member 30xe2x80x2 and 30xe2x80x3 and annular resilient member 32xe2x80x2 and 32xe2x80x3 located between the inner and outer members. As shown in FIG. 1, the resilient member has a constant thickness as it extends axially along axis 25 between the inner and outer bearing members. The constant thickness is illustrated in the enlarged detail view FIG. 2 and is referred to in the Figure as xe2x80x9ctxe2x80x9d. Each pitch bearing 22xe2x80x2 and 22xe2x80x3 is seated on a respective sliding bearing surface 23 and 27. During use, the bearing may be displaced in small axially directed distances along the sliding bearing surfaces.
When elastomeric bearings are loaded relative to elastic center 20 by applied moment 26, an unbalanced load reaction and differential stress/strain occur in constant thickness resilient layers 32xe2x80x2 and 32xe2x80x3. As shown in the schematic representation of body 12 and bearings 22xe2x80x2 and 22xe2x80x3 in FIG. 3, the loading at outer bearing ends 35A and 35B is greater than the loading at inner bearing ends 36A and 36B. The different effective loads are represented schematically in FIG. 3 by differing the lengths of the tails of the load arrows. The greatest loads have the longest tails and the smallest loads have the shortest tails. As shown by the schematic representation of the reaction load distribution in FIG. 3, the load reaction (pressure distribution) increases in the axial direction along axis 25 from end 36A, 36B to end 35A, 35B as one moves farther away from the applied moment 26. As a result, the inner and outer sleeves 28 and 30 will compress or pinch the elastomer layer 32 more at outer ends 35A and 35B than at the inner ends 36A and 36B. causing the outer portions of the bearing sleeves to wear out prematurely. The uneven load distribution provides uneven wear on the bearings and causes the outer portions of the bearing sleeves 23 and 27 to wear out prematurely. The portions of bearing surfaces 23 and 27 at ends 35A and 35B that experience the greatest loading wear out sooner than the portions of the bearing surfaces at ends 36A and 36B.
The elastomer portions at ends 36A and 36B wear out over period of use that is longer than the period it takes for elastomer portions at ends 35A and 35B to wear out. In order to maximize the useful life of the pitch bearings 22xe2x80x2 and 22xe2x80x3 it would be desireable to provide constant or even wear of the sliding bearing surface of each of the pitch bearings.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative pitch bearing is provided including features more fully disclosed hereinafter.
In one aspect of the present invention this is accomplished by providing an elastomeric bearing which produces substantially constant bearing wear along the length of the sliding bearing surface. The bearing of the present invention comprises a bearing that includes a first bearing support member; a second bearing support member spaced from the first bearing support member, the first and second bearing support members each having a first end and a second end; and a resilient member between the first and second bearing support members, the resilient member having a thickness, the thickness varying between the first and second ends of the bearing support members, and whereby the bearing provides for constant bearing wear along the length of the bearing. The bearing may be a pitch bearing for use in a pitch hinge in a helicopter or may be used in an articulated vehicle.
In the bearing of the present invention the thickness of the resilient member varies linearly between the first and second bearing ends. Depending on the bearing loading the thicker end of the resilient member may be located at either the first or second bearing end. In the present invention bearing, the first bearing support member, the second bearing support member and the resilient member are annular and the thickness of the resilient member varies in the radial direction between the first and second ends. The first and second bearing support members have inner surfaces that are tapered inwardly and converge at the first end and the taper defines an angle that is between four and six degrees.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.